How to get RGB values for a portion of texture, specifically an image
If you're just trying to extrude a silhouette of an object, doing it with a subdivided plane is probably the worst option (you'll end up with potentially useless geometry, I'm assuming this isn't what you want).

Your best bet here is probably to import the image as a background image through this interface in the right panel.



Then go into an orthographic camera mode and create geometry over it.

Another option could be to open the picture in a program like Adobe Illustrator, automatically or manually trace it to create a path, export as SVG and import to use as geometry in Blender.